1. Field of the Invention
The present invention relates to a method for the preparation of high solids silicone resin coating solutions. More specifically, the invention is a method for concentrating a silicone resin solution to provide a higher solids level for improved silicone resin performance.
2. Description of Related Art
Silicone resin coating compositions are presently being utilized to protect the surface of wood, metals, and synthetic polymers. These transparent coatings provide excellent mar and scratch resistance to acrylic and polycarbonate sheets used as glazing. An example of a widely used transparent polycarbonate glazing on which these coating are useful is Lexan® polycarbonate resin sold by General Electric Company.
The silicone resin coating formulations typically comprise an aqueous dispersion of colloidal silica or silica gel and silicone silsesquioxane resins formed from hydrolizable silanes, such as the trifunctional silanes of the formula, R′Si(OR)3 and a solvent medium such as alcohol and water.
Degradation of silicone coating compositions, especially for weatherable coatings, from prolonged exposure to moisture, humidity and ultra violet light during outdoor use, has presented certain problems. Yellowing and hazing along with delamination of these protective coatings is often observed. Attempts to enhance the longevity of such silicone coatings have included modification of the solvent carrier, and modifications of the resin composition. These modifications include the introduction of additives to the coating as disclosed in U.S. Pat. No. 4,277,287 and altering the pH of the composition as disclosed in U.S. Pat. No. 4,368,235.
Control of coated film thickness is a key process parameter in application of coatings in order to achieve desired performance. Higher solids levels are often needed in order to increase the thickness of the final coated film when applied to a given substrate. The silicone resin compositions to which the claimed method can be applied can be found in U.S. Pat. No. 3,976,497, 3,986,997 and 4,177,315.
Traditional methods for increasing solids of a silicone resin solution require either, the adjustment of the reactant stoichiometry in order to minimize the formation of byproduct alcohols and residual water or, the application of heat and/or vacuum to “strip” off solvents and concentrate the resin. The former method (stoichiometry) results in a compromise in other performance attributes, such as, hardness of the cured coating and resistance to cracking, because the inputs into the silicone resin are changed to minimize the byproducts of the reaction. The later method, i.e. stripping, is practiced to manufacture select silicone hardcoat grades. In this method, the volatile solvents present in the initial resin hydrolyzate are stripped off to give an intermediate solution that is higher in resin content (>35%). Care must be exercised when utilizing this method as aggressive application of heat/vacuum or applications of heat/vacuum for long periods of time, i.e >24 hours, can result in significant degree of silanol functional group condensation. These functional groups are required to effect cure of the coating after it is applied to the substrate. Moreover, excessive condensation of silanol groups negatively impacts properties such as adhesion and hardness of the cured coating.
While the procedures mentioned above have provided acceptable coating formulations, there still remains room for improvement. For example, the method comprising this invention is a significant improvement over the currently practiced processes as it does not require the use of heat or vacuum to concentrate silicone resin solutions, and takes significantly less time to effect the increase in concentration of solids in the resins.